Research On Sound Radiation Properties Of Complex Vibration Modes In Viscoelastic Damping Composite Structure

With the strategy of sustainable development,engineering manufacturing began to develop towards the design of lightweight and high-strength structures.However,it comes the contradiction between the lightweight design of the structure and the requirements of noise and vibration.The typical composite structure with viscoelastic damping material as the core has excellent damping characteristics to reduce vibration and noise.Therefore,it has been widely used in the engineering field.In this thesis,two aspects of research work are conducted.On the one hand,the viscoelastic damping analysis model with loss factor and elastic modulus frequency dependent characteristics is established by the finite element iteration method.The acoustic radiation characteristics and mechanism of free damping and constrained damping structure are further proposed and compared from the complex mode aspect.The evaluation indexs which quantify the degree of modal complexity of viscoelastic damping are obtained.The damping effect,complex mode complexity,complex mode self-radiation efficiency,sound power and underwater sound radiation characteristics of free damping and constrained damping structure with different viscoelastic damping thickness,covering area ratio and laying position are compared,and the influence of modal complexity on sound radiation is discussed.The research results provide a useful reference for the application of viscoelastic damping in structural vibration and acoustic radiation control.On the other hand,in view of the cross-modal coupling effect between the structural vibration modes,the method of calculating the zeroes and poles of mutual-radiation sound power is proposed,which provides a useful reference for structural acoustic radiation analysis and control by using vibration mode.The research contents include:The accuracy of finite element iteration method was verified by simulating the viscoelastic material loss factor with frequency varying characteristics.The different viscoelastic damping composite structure analysis model is established by using the finite element iteration method.The modal analysis of the constrained damping plate and the corresponding free damping plate with different viscoelastic damping thickness,covering area ratio and laying position is carried out to obtain the inherent properties such as complex mode and damping ratio.In this paper,the existing modal complexity indexes are compared with each other under the different damping positions.The effectiveness and universality of the modal complexity indexes are analyzed by taking Mode Shape Complexity(MSC)parameters as reference.The evaluation indexs which quantify the degree of modal complexity of viscoelastic damping is obtained.The modal complexities of constrained damping plate and free damping plate under different damping laying modes are calculated,which indicates that the different distribution of viscoelastic damping layer materials such as thickness,covering area ratio and laying position will lead to different complex modes.The self-radiation efficiency of different distribution of viscoelastic damping layer materials is presented.The self-radiation efficiency of the complex mode is compared with the corresponding real mode without any damping treatment plate to analy the effect of complex mode on radiation efficiency of free damping plates and constrained damping plates.The Complex Mode Radiation(CMR)parameter,proposed to quantify the impact on sound radiation by the complex vibration,represents the integral between the real mode and the corresponding complex vibration mode in a certain frequency band.The higher the absolute value of CMR,the greater the influence of the complex mode on the acoustic radiation efficiency.The Complex Mode Radiation Index(CMRI1)was improved to quantifies the impact on sound radiation normalised by the level of complexity in complex vibration modes.The acoustic radiation of different distribution of viscoelastic damping layer materials is computed in air and underwater to analy the effect of complex mode on radiation sound power.The variation of sound power of free damping structure and the corresponding constrained damping structure with different viscoelastic damping thickness,covering area ratio and laying position is compared.The normalised damping ratio,mode complexity and Complex Mode Radiation Complexity(CMRC)indexes were compared with the proposed CMRC1 indexes.The comparison shows that CMRC1 can accurately quantify the impact on sound power in complex vibration modes.The cross-modal coupling effect between the structural vibration modes of a simply supported rectangular panel on the radiated sound power is presented.An analytical solution to zeroes and poles of modal coupling velocity is obtained.Then numerical example and analysis are given to demonstrate the coupling effect.The zeroes of the modal coupling velocity at which the cross-modal coupling terms have no effect on the radiated sound power are the same as the zeroes of mutual-radiation sound power and are close to the natural frequencies of the structure,so the effects of the cross-modal coupling tend to be more remarkable at a non-resonance frequency.The poles of the modal coupling velocity at which the cross-modal coupling contributes significantly to the sound field are calculated in the numerical example.